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Topic: My home-made ferrite antennas (Read 54315 times)

I found a source for some 7.5mm x 50mm ferrite rods with an initial permeability of 2000u - ideal material for a ferrite antenna, but too short to be of practical use. However, the price was very cheap! I ordered 32 of them to experiment with. I decided to try to make some 7.5mm x 250mm cores by gluing 5 of the smaller rods end-to-end. Gluing ferrite in this manner is an accepted practice and has a very small effect on the inductance of the final antenna.

Richo suggested that close-winding a single layer of AWG26 enameled wire on a 10mm x 200mm core, leaving 5mm unwound at the ends, would produce a ferrite antenna with outstanding performance. I figured that a 7.5mm x 250mm rod would be equivalent.

Here is a photo of the 50mm rods and the "Super-thick" commercial grade cyanoacrylate glue I used to assemble the longer rods. The salt shaker is not a part of the procedure!

One drop in the center of the rod is enough.

I used a piece of aluminum channel clamped into a table vice and lined with waxed paper to align the rods as they were glued together. The waxed paper prevents sticking to the aluminum. 30 seconds of pressure was applied to allow the glue time to set.

The third rod ready to be glued...

The final 250mm assembly of 5 rods. I allowed 30 minutes before removing the assembly from the channel. I allowed another 2.5 hours for the glue to fully cure before winding the cores.

This photo shows the first finished antenna. The cores were wound with 26AWG enamled copper wire. I used electrical tape to secure the ends and sections of the coil as I wound to prevent unraveling and to keep the windings tight. When I reached the far end, I loosely wound the free end back to the other end. This is the end of the coil that will be connected to ground, following the example of the commercial antenna provider.

A close-up of the finished first antenna and extra cores...

The second antenna completed and the two additional cores for a second antenna pair.

A close-up of the two completed antennas.

I slipped the antennas into foil-covered and grounded 3/4" PVC conduit sections with a longitudinal gap in the foil to prevent shorting the magnetic field. These things have incredible gain! I am still testing, but they appear to be giving my high-gain flat-panel loop some serious competition!

The cores look to be hollow. I would run a wooden dowel thru them prior to gluing (they might not even need gluing)them together this should make them considerably stronger and less prone to break. I don't think it would have any if much effect on their Q.

That is an excellent idea. I re-glued the cores with epoxy last night after cleaning up the ends with alcohol. The joints seem strong, but I have the feeling that ANY glued joint is going to be inherently weak. Ferrite is a powder mixed into a binder. The brittle composite nature of ferrite does not have enough strength to support the glued joint.

There is sufficient strength to support winding the coils, but shear strength is low. Pre-wrapping the cores with shrink wrap would stabilize the glued joints laterally. In addition, spacing the windings away from the core should minimize the distributed capacitance and self-resonance.

Darn, the long pieces of shrink wrap I have are just a hair to small to get the 7.5mm rods through. Off to Lowes!

Got my kit of parts yesterday. My youngest son was ecstatic since it came in a German language box with a cool clock on it.

He was crushed when I opened it. Now I'll have to come up with some sort of consolation prize for him.

I'm very happy.

Nonetheless, IF one could find shrink tubing just the right size to get the cores in, why not put the assembly together without any glue at the joints at all, slightly compress the whole stack and then shrink the tube assembly? No glue, tightly held, and apparently the function of a rod built up from many pieces, as long as the ends are in close approximation, don't need any sort of fixation to conduct the magnetic lines through them, and glue is superfluous to function.

JB Weld might be stronger, but then again the problem is that cores are inherently fragile already, and to expect the last few millimeters of a compressed substance to hold tight is really asking a lot. I'm still interested in the idea of using a dremel or such to make a little dimple or 5mm hole into the rod so that a load capable glue like epoxy can get more area of purchase to adhere to and distribute the load from the rod next to it through more of an area than just those smooth surfaces.

Darn, I should have ordered MORE of those rods when the order went in.

I just wrapped each epoxied joint tightly with electrical tape, rewound the cores, wrapped the assembly in thin foam packing and stuffed it in the shielded PVC for a tight friction fit. I'm finishing up the wiring and enclosure assembly now.

Got my kit of parts yesterday. My youngest son was ecstatic since it came in a German language box with a cool clock on it.

He was crushed when I opened it. Now I'll have to come up with some sort of consolation prize for him.

I'm very happy.

Nonetheless, IF one could find shrink tubing just the right size to get the cores in, why not put the assembly together without any glue at the joints at all, slightly compress the whole stack and then shrink the tube assembly? No glue, tightly held, and apparently the function of a rod built up from many pieces, as long as the ends are in close approximation, don't need any sort of fixation to conduct the magnetic lines through them, and glue is superfluous to function.

JB Weld might be stronger, but then again the problem is that cores are inherently fragile already, and to expect the last few millimeters of a compressed substance to hold tight is really asking a lot. I'm still interested in the idea of using a dremel or such to make a little dimple or 5mm hole into the rod so that a load capable glue like epoxy can get more area of purchase to adhere to and distribute the load from the rod next to it through more of an area than just those smooth surfaces.

Darn, I should have ordered MORE of those rods when the order went in.

There has been no degradation in performance with the re-glued rods. I do think that shrink-wrapping the glued rods before winding or the entire antenna assembly after winding. will provide excellent mechanical stability. Even if the glued joint were to fracture within the shrink tubing, I do not think performance would be affected at all, as the ends would still be held tightly together, as long as the shrink tubing extended past the ends of the assembly to provide pressure longitudinally.

I found 25 feet of 3/8 inch blue shrink wrap on Ebay for about $8.00 with free shipping. This is a great price. Lowes wanted $35.00 for 48 inches of the stuff! Lots of colors are available at this price. I'll use this stuff to protect and stabilize the assemblies when it arrives. I plan on making 3 sets of antennas from the 32 rods I ordered.

Egon warned me about unwanted self-resonance in the 10KHz to 100KHz area if I close-wound the wire directly on the ferrite. However, I looked at Richo's PDF file with frequency response plots of various ferrite antennas, including one very similar to mine (20CM, close wound to within 5mm of each end, directly on the ferrite). The response was perfectly flat across the 10KHz to 100KHz band. Self-resonance disturbances did not appear until much higher frequencies.

I received the shrink tubing for the ferrite antennas today. Following advice from the forums, I decided to encase the completed ferrite assemblies, re-glued with epoxy, in shrink wrap. I used 3/8 inch tubing which was just wide enough to slip the entire assembly inside easily. I then shrunk each end and the center portion, to insure that the ferrite segments were squeezed together by the wrap longitudinally. This ended up working really well. The final assembly is quite secure and break-resistant.

This is one of the epoxied re-glued cores, almost finished winding.

Just a few mm to go. The epoxied and taped joint can be seen beneath the winding.

Components of the ferrite antenna housing. The antenna assemblies are wrapped in thin foam and friction-fit in the tubes. The tubes are tapped into the "L" box, where the connections to the shielded cabling to the amp are made. The tubes are friction fit into the "L" with a thin coat of silicone grease to ease assembly/disassembly. The same was done with the end caps.

Very professional looking job. 2 questions with the metal shielding, do you ground the shields to the circuit ground and is it important or does it matter the direction that the slot in the shield points when mounted? Thanks.

The shields are tied to one side of the ferrite winding (the return wire from the winding end). Both wires are connected to the grounded side of the amp input, which is in turn grounded through the shielded Ethernet cable to the controller board and its ground.

Very professional looking job. 2 questions with the metal shielding, do you ground the shields to the circuit ground and is it important or does it matter the direction that the slot in the shield points when mounted? Thanks.

A L O T of work to get these photos for the rest of us to look at and see what things other minds have thought of with fabrication that would have taken more time to figure out on our own, or serve as a jumping off point to modify as we have to with available materials.

What was the search you used for the heat shrink? I find several, but wonder if you remember the vendor you chose?

"Live rubber" is thin rubber foam sheeting that hobby shops sell to wrap model airplane fuel tanks in to prevent the fuel from foaming with engine vibration. Dubro makes and sells it in two-sheet packages. The stuff is hard to describe, except it is very spongy and resilient. It makes a great shock absorbing material.